Engine/Transmission (1994 - 1998) transmission Not Shifting When Passing

[GROBBINS]

When passing or heavy acceleration, transmission won't shift until I let off the throttle. Then it kicks down, but only after I let off. With normal acceleration the trans shifts fine. Any insite?

 

[Howard Durand]

I would guess tht TPS might be the problem but why would you want it to kick down when passing? The kickdown is nice when pulling hard and you need to get the revs up but a downshift when passing leaves you in a real vulnerable position. High engine rpm but no added vehicle speed. I learned that the first trip I made with mine.

Since I replaced the TPS on mine I no longer have a kickdown out of TC lockup and I like it that way.

 

[GROBBINS]

Vulnerable is exactly the position I was in, when I accelerate to pass it won't shift into a higher gear. I accelerate a few mph, but mostly just get engine revs, until I let off the accelerator, then it shifts down to a higher gear with the torque converter locking up and putting power to the wheels. Making passing not even an option. Is the TPS accessable from below, after dropping the pan?

 

[Howard Durand]

First of all, is this a problem that has just started or has it always been that way? If you can depend on it to not shift until you lift the throttle, that is not such a bad deal. The killer is to have it "kick down" just when you get alongside someone on a two lane highway. There you sit with engine rpm at max and losing ground.



The TPS, (throttle position sensor) is a variable resistor device found under the hood. It is mounted on the side of the injection pump directly below the #1 injection valve. It is about 1. 5" square, black, and has a three wire cable plugged into it that runs to the PCM.

It is mechanically connected to the throttle mechanism in such a way that it is rotated as the throttle is advanced. This provides a changing voltage signal to the PCM that determines when the transmission shifts into overdrive.

It may or may not be part of your problem but should be checked to see if it is providing the right voltage signal to the PCM.



I don't think the TC lockup delay under heavy acceleration is all that unocommon. I sometimes have to lift the throttle a bit to get mine to lock up.

My transmission is the 47RH while yours is the 47RE which operates somewhat differently.

You should have the TPS output voltage checked first as they seem to be a trouble maker for these trucks.

They are somewhat expensive for what you get and you might want to investigate some other alternatives before replacing it.

Search some of the threads on the forum for TPS problems and solutions as well as alternatives.



I have replaced my TPS with a manually adjustable potentiometer and am quite satisfied with it. I do have to lift the throttle sometimes to get lockup but it will not kick down no matter how hard I mash the accellerator. I first thought that this might be a problem but find it to be a real benefit.

It will still unlock with a tap on the brake or when engine speed gets below 1200 rpm.

 

[GROBBINS]

Howard, it is a new problem after several thouand miles on a new trans filter. I had a problem a year ago with my TC slipping in and out of lock up at 50-55mph with gradual acceleration. I cleaned all my connections and changed my trans fluid & filter, installed ams oil synthetic and it shifted like a dream until now. With the problem before, the TPS was a factor, I haven't checked the voltage yet. But it sounds like things are pointing in its direction. Thanks for the info. Its always nice for me to learn more about my rig. Gabe